1. Field of the Invention
The present invention relates to a phase-shifting photomask blank and a phase-shifting photomask for use in a projection exposure apparatus, and methods of manufacturing such a phase-shifting photomask blank and a phase-shifting photomask, and more particularly to a phase-shifting photomask blank and a phase-shifting photomask which have a phase-shifting film whose composition varies in the transverse direction thereof, and methods of manufacturing such a phase-shifting photomask blank and a phase-shifting photomask.
2. Description of the Prior Art
One process of manufacturing a conventional phase-shifting photomask will be described below with reference to FIGS. 1(a) through 1(c) of the accompanying drawings. First, as shown in FIG. 1(a), a phase-shifting layer 104 of molybdenum silicide oxide nitride (MoSiON) is grown on a surface of a transparent substrate 103 of quartz. Then, as shown in FIG. 1(b), a protective layer 105 of molybdenum silicide oxide nitride, which has a different composition from that of the phase-shifting layer 104, is deposited on the phase-shifting layer 104. The phase-shifting layer 104 and the protective layer 105 jointly serve as a phase-shifting film 100. The phase-shifting film 100 and the transparent substrate 103 make up a phase-shifting photomask blank 101. Thereafter, as shown in FIG. 1(c) the phase-shifting film 100 is selectively etched away by dry etching, using a patterned photoresist film 108 (see FIG. 3 of the accompanying drawings) on the protective layer 105. After being thus selectively etched, the phase-shifting film 100 has openings 106 through which the surface of the transparent substrate 103 is exposed and lands positioned as shifters 107 between the openings 106. The openings 106 and the shifters 107 jointly make up a desired circuit pattern. The assembly thus formed serves as a phase-shifting photomask 102.
The phase-shifting film 100 is generally required to have a sufficiently high light transmittance with respect to exposure light in a short wavelength range, e.g., of a wavelength of 248 nm, used to achieve a highly accurately circuit pattern. At the same time, the phase-shifting film 100 is also required to have a sufficiently low light transmittance with respect to light in a longer wavelength range, e.g., of a wavelength of 488 nm, used to inspect the phase-shifting film 100 for any defects.
An exposure step in the process of manufacturing the phase-shifting photomask is accompanied by cleaning the phase-shifting photomask with a chemical. Therefore, the protective layer 105 is required to be highly resistant to the chemical used, and also to be capable of adjusting the phase angle of the phase-shifting layer 104 to achieve a desired phase angle for the phase-shifting film 100 in its entirety.
A photolithographic process effected on a semiconductor wafer, for example, using the phase-shifting photomask 102 will be described below with reference to FIGS. 2(a) through 2(c) of the accompanying drawings. As shown in FIG. 2(a), rays of exposure light 110 are applied to the reverse side of the phase-shifting photomask 102, transferring the circuit pattern on the phase-shifting photomask 102 onto the semiconductor wafer based on the difference between intensities of the exposure light 110 that has passed through the openings 106 and the shifters 107.
If the exposure light 110 that has passed through the openings 106 and the exposure light 110 that has passed through the shifters 107 are out of phase with each other by .pi. (rad.), as shown in FIG. 2(b), then the intensity of the exposure light 110 that reaches the surface of the semiconductor wafer is nil in the vicinity of the boundaries between the openings 106 and the shifters 107 as shown in FIG. 2(c). Consequently, it is possible to transfer a highly accurate, fine circuit pattern from the phase-shifting photomask 102 to the semiconductor wafer.
It is known that the phase-shifting angle .phi. (rad.) of the phase-shifting photomask 102 is expressed by: EQU .phi.=2.pi..multidot.(n.sub.1 -n.sub.0).multidot.d.sub.1 .lambda.+2.pi..multidot.(n.sub.2 -n.sub.0).multidot.d.sub.2 /.lambda.(1)
where n.sub.0 is the refractive index of a medium (which air in normal usage), refractive index n.sub.0 of air being 1, .lambda. is the wavelength of exposure light, d.sub.1 and d.sub.2 are the thicknesses of the phase-shifting layer 104 and the protective layer 105, respectively, and n.sub.1 and n.sub.2 are the refractive indexes of the phase-shifting layer 104 and the protective layer 105, respectively. The thicknesses d.sub.1, d.sub.2 of the phase-shifting layer 104 and the protective layer 105 are selected such that the phase-shifting angle .phi. is .pi. at the wavelength .lambda. of the exposure light that is actually used.
While the phase-shifting film 100 of the above conventional phase-shifting photomask 102 is composed of the two layers 104, 105, the phase-shifting film of another phase-shifting photomask may be composed of three or more layers. At any rate, the light transmittance of the phase-shifting film is normally in the range of from 4% to 20% in order to produce an adequate intensity of exposure light for photolithography and adjust the film thickness, after being developed, of a resist film coated on the semiconductor wafer.
For satisfying both desired optical characteristics and chemical-resistance requirements of the phase-shifting film, it has been the general practice to construct the phase-shifting mask of either a number of layers having different compositions or a thin film whose composition gradually varies, rather than a film having a single composition.
However, when a number of layers having different compositions or a thin film whose composition gradually varies is patterned into a desired circuit by dry etching, the produced circuit pattern tends to be inaccurate in shape or lack pattern sections, resulting in defective phase-shifting photomasks.
A study to find causes of such inaccurate circuit pattern shapes or lost pattern sections in the conventional phase-shifting film 100 has indicated that when the phase-shifting film 100 is selectively etched by dry etching to produce the shifters 107, each of the shifters 107 is recessed centrally in its transverse direction as shown in FIG. 3 and has a concave side surface which does not lie perpendicular to the transparent substrate 103.
The recess in the concave side surface is produced because of different rates of side etching of the phase-shifting film 100 in its transverse direction. Specifically, the layers of the phase-shifting film 100 are subjected to side etching at different rates, respectively. The concave side surface of the phase-shifting film 100 tends to lower the controllability of the light transmittance of the phase-shifting film 100 at an end of each of the shifters 107, and make the end of each of the shifters 107 and hence the circuit pattern inaccurate in shape. Furthermore, since the overhanging region above the recess in the concave side surface can easily be broken at the end of each of the shifters 107, the circuit pattern is liable to lose pattern sections.
Consequently, there have been demands for finding solutions for such problems.